Conventionally, in a mobile radio communication system and the like, a CELP (Code Excited Linear Prediction) scheme has been widely used as an encoding scheme for speech communication, since speech signals can be encoded with high quality at relatively low bit rates (about 8 kbit/s in telephone band speech). Meanwhile, in recent years, speech communication (VoIP: Voice over IP) using an IP (Internet Protocol) network is rapidly becoming widespread, and it is foreseen that the technique of VoIP will be used widely in the mobile radio communication system.
In packet communication typified by IP communication, since packets are sometimes lost on the transmission path, a scheme that is robust against frame loss is preferable as a speech encoding scheme. Herein, in the CELP scheme, since a current speech signal is encoded using an adaptive codebook that is a buffer of an excitation signal that was quantized in the past, when an error once occurs on the transmission path, the contents of the adaptive codebook on the encoder side (transmission side) and the decoder side (reception side) fail to be synchronized, and the error influences not only the frame where the error occurs on the transmission path, but also subsequent normal frames where the error does not occur on the transmission path. Therefore, the CELP scheme is not regarded as being very robust against frame loss.
As a method of enhancing the robustness against frame loss, for example, a method is known of performing decoding using another packet or a part of a frame when a packet or apart of the frame is lost. Scalable encoding (also referred to as embedded encoding or layered encoding) is one of techniques to implement such a method. The information encoded with the scalable encoding scheme is made up of core layer encoded information and enhancement layer encoded information. A decoding apparatus that receives the information encoded with the scalable encoding scheme is capable of decoding a speech signal that is at least essential to reproduce speech by using only the core layer encoded information even without the enhancement layer encoded information.
As an example of scalable encoding, there is an encoding scheme having scalability in frequency band of a signal which is target of encoding (for example, see Patent Document 1). In the technique as described in Patent Document 1, a down-sampled input signal is encoded in a first CELP encoding circuit, and the input signal is further encoded in a second CELP encoding circuit using an encoding result in the first circuit. According to the technique as described in Patent Document 1, by increasing the number of encoding layers and increasing a bit rate, it is possible to increase the signal bandwidth and improve the quality of a reproduced speech signal, and it is thus possible to decode a speech signal with narrow signal bandwidth in an error-free state and reproduce the signal as speech even without the enhancement layer encoded information.
Patent Document 1: Japanese Patent Application Laid-Open No. HEI11-30997